The long-term objectives of this project are to elucidate the neural and hormonal bases of learned vocal behavior in passerine birds. We have previously shown that lesion of a hormone-sensitive forebrain nucleus (the magnocellular nucleus of the anterior neostriatum, MAN) in male zebra finches disrupts song learning during a critical period of development in zebra finches, but has no effect on production of already-learned song patterns of adult birds. We have also discovered that approximately half of the neurons originally contained within MAN are lost during this period of development, and that there is an increase in the ability of individual MAN neurons to concentrate androgenic hormones during the same time. Experiments to be conducted will investigate: (1) The exact time course of the change in androgen accumulation within MAN cells, as well as the axonal connections of androgen target cells. These experiments will help to determine whether hormone accumulation in MAN cells is importantly involved in the change in function of MAN, the loss of MAN cells, and the pattern of axonal connections made and received by MAN neurons. (2) Neuro-anatomical tract-tracing experiments will examine the exact time course of thalamic innervation of MAN neurons. Previous data have indicated that thalamic axons grow into and innervate MAN relatively late in development, and may therefore be related to specific aspects of vocal learning. (3) Systemic treatment of juvenile males with anti-androgenic and/or anti-estrogenic drugs will be used to directly test the role of gonadal hormones in brain development and vocal learning. (4) We will examine the extent of sex differences in axonal connections within the song-control system, and assess the influence of perinatal treatment with steroid hormones in organizing these neural circuits. (5) We will make lesions of various song-control nuclei at different stages of vocal development. These data will determine whether other nuclei exert varying degrees of control over song behavior at different times of development, and will assess the potential role of synaptic competition between different song-control regions. (6) We will begin a series of studies designed to examine the role of vocal muscles in sexual differentiation of the song-control system. These findings will provide specific hypotheses of mechanisms of vocal learning that can be applied to humans, and may have important implications for problems of vocal perception and production in humans.